Interactive practical

Principle of moments

Balance a metre rule on a pivot, slide masses until it is level, compare clockwise and anticlockwise moments, and find an unknown mass.

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GCSE Physics

Verifying the Principle of Moments

WJEC 3420QS — matched to your course.

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Practical focus

Use a balanced beam and known forces to compare total clockwise and anticlockwise moments.

Practical method

Aim

Verify the principle of moments: for a balanced beam, total clockwise moment equals total anticlockwise moment.

Variables

Independent variable: forces and their distances from the pivot
Dependent variable: whether the beam balances (and the distance needed to balance)
Control variables: the same metre rule balanced at its centre of mass; distances measured from the pivot; masses hung vertically.

Apparatus

Metre rule
Pivot (knife edge or stand with a hole through the rule's centre)
Slotted masses on hangers (or weights in N)
Thread loops
Newtonmeter

Method

Balance the metre rule on the pivot at its centre of mass (about the 50 cm mark).
Hang a 1.0 N weight 30 cm to the left of the pivot.
Hang a 1.5 N weight on the right and slide it until the rule balances; record its distance from the pivot.
Calculate the anticlockwise moment (1.0 × 30) and clockwise moment (1.5 × d).
Repeat with at least four different combinations, including two weights on one side.

Sample results

Rule pivoted at its centre of mass; distances from the pivot.

Anticlockwise force × distance / N cm	Clockwise force × distance / N cm	Anticlockwise moment / N cm	Clockwise moment / N cm
1.0 × 30	1.5 × 20	30	30
2.0 × 15	1.0 × 30	30	30
1.5 × 40	2.0 × 29	60	58
1.0 × 20 + 1.0 × 40	2.0 × 30	60	60

Analysis

In each balanced arrangement the clockwise and anticlockwise moments are equal within experimental uncertainty (for example 60 and 58 N cm). This supports the principle of moments. Moments in N cm can be converted to N m by dividing by 100.

Errors and improvements

Problem	Effect	Improvement
Rule not balanced at its centre of mass	Rule's own weight adds a moment	Pivot the rule at its centre of mass first
Loops slipping or not vertical	Distances wrong	Use thin thread loops and check they hang vertically
Judging balance by eye	Small imbalance unnoticed	Use a reference mark or spirit level

Risk assessment

Hazard	Risk	Control measure
Falling masses	Injury to feet	Keep feet clear and use a cushion
Rule tipping off the pivot	Masses falling	Add masses gently

Common mistakes

Measuring distances from the end of the rule instead of the pivot.
Forgetting to add moments when two weights are on one side.
Mixing grams and newtons.

Worked analysis

A 2.0 N weight hangs 15 cm left of the pivot. A 1.0 N weight balances it on the right. Anticlockwise moment = 2.0 × 15 = 30 N cm, so the 1.0 N weight must be 30 cm from the pivot.

Common misconception

A beam balances when moments are equal, not when the forces are equal. A small force far from the pivot can balance a large force close to it.

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